Simplification and cost reduction in production processes have been in positive progress coupled with an eco-friendly low energy orientation. Especially, production processes of optical, electric and electronic devices and displays mostly involve high temperature heating steps requiring vast amounts of energy, time and equipment so as to cure adhesive, sealing and embedding materials, thus requiring improvement. This improvement for the heating steps has great meaning not only for energy and cost but also for production techniques of not damaging other materials.
Recently, in order to solve these problems, attention has been paid to UV-curable compositions. The UV-curable composition contains photoinitiators that can be activated by UV irradiation, whereby polymerization or crosslinking reaction proceeds to permit curing within a short time of several tens of seconds to ten and several minutes. For this, other members are hardly damaged, and no large-sized equipment is necessary. In recent years, UV irradiators making use of light emitting diodes (LEDs) have been developed, whereby excellent production processes have been performed.
With regard to the UV curable silicone compositions, a method using photocationic polymerization (see Patent Document 1: JP-A 2008-195931), and a method using radical polymerization (see Patent Document 2: JP 3894873) have been proposed. In the former method, an onium salt capable of generating an acid by UV irradiation is contained in the composition. If the composition is used on an electric or electronic substrate, there may be some concern that the substrate undergoes corrosion. The latter method has a feature in that the reaction velocity becomes high owing to high reaction activity and thus curing is achieved within a short time. However, the radicals have a very short life and are readily deactivated with oxygen. Eventually, curability on the composition surface in contact with air may considerably lower.
To cope with the above-described problem on cure inhibition, a variety of sensitizers have been studied as an additive (see, for example, Patent Document 3: JP-A 2001-064593, Patent Document 4: JP-A 2005-040749, and Patent Document 5: JP-A 2013-253166).
In the method described in Patent Document 3, some kind of amine compounds is added as an additive so as to impart basicity thereby enhancing reactivity of a photoinitiator or (meth)acrylate double bond. In the method described in Patent Document 4, an anthracene derivative or xanthone derivative having a high molar absorbance coefficient over a wide range of wavelengths is allowed to coexist thereby enabling a higher excitation energy to be released from irradiation light.
The method described in Patent Document 5 makes use, as a curing sensitizer, of a (meth)acrylate obtained from an alcohol having one or more hydroxyl groups and two or more oxygen atoms in the molecule so as to enhance polymerization reactivity.
However, in the methods of Patent Documents 3 to 5, although the respective reactivities can be improved, storage stability becomes worsened with the problem that curing gradually proceeds in sensitizer-coexisting conditions even in cold and dark storage. Moreover, where an amine compound is added as an additive, a problem is involved in its discoloration. With the anthracene derivative or (meth)acrylate described in Patent Document 5, a problem is that its solubility in silicones is low.